Engine control unit

ABSTRACT

An engine control unit for a gas fuelled internal combustion engine detects engine speed, throttle position, manifold absolute pressure, gas pressure, gas temperature, battery voltage, air temperature, engine phase and boost pressure control valve feed back position. The control unit calculates a percentage full load value for the engine based upon the detected engine speed and throttle position and utilizes the calculated percentage full load value to calculate the injector on time for each gas injector in a gas delivery system for the engine. Typically, the control unit employs the percentage full load value and the engine speed to calculate a required manifold absolute pressure value, and this calculated manifold absolute pressure value may then be employed together with the percentage full load value to calculate a percent allowable load value. The engine control unit employs the percentage allowable load value to calculate injector on time and spark advance for the engine.

FIELD OF THE INVENTION

The present invention relates to an Engine Control Unit (ECU) for a gasfuelled internal combustion engine, and relates particularly, though notexclusively, to an ECU for a spark ignition gas fuelled engine convertedfrom a turbocharged diesel engine.

BACKGROUND TO THE INVENTION

In the prior art, naturally aspirated diesel engines have been convertedto operate on a gaseous fuel, for example natural gas, using carburettedtechnology. However, problems have risen with the use of gascarburettors, particularly when using compressed natural gas (CNG) as afuel. Furthermore, the advent of turbocharged diesel engines has createdfurther difficulties for most engine converters as the engines are morehighly stressed.

The temperature of natural gas after passing through the pressureregulators has been measured to be as low as -40° C. when operating withfully charged CNG cylinders. Gas carburettors deliver fuel on a volumebasis rather than a mass (or heat value) basis, and therefore there is atendency for the carburettors to overfuel the engine. Thischaracteristic affects both naturally aspirated and turbocharged engineswith gas carburettors.

Problems also arise due to the design of the intake manifold andcarburettor location which can result in some cylinders receiving moreor less gas than other cylinders. This causes a tendency for somecylinders to detonate under certain load conditions. Attempts to controlthis by the use of detonation sensors have not been entirely successful.

A further problem with turbocharged engines is the inability to controlthe performance of the turbocharger since it is essentially a "freeagent" which is not controlled directly by the throttle. Therefore it isalso difficult to measure its performance and adjust the fuel flow andboost pressure accordingly.

The Engine Control Unit of the present invention was developed with aview to overcoming one or more of the above-noted problems in the priorart.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of controlling the operation of a gas fuelled internal combustionengine, the engine having a gas injector for injecting gaseous fuel intoeach cylinder, the method comprising the steps of:

detecting the current throttle position of the engine;

calculating a percent full load value for the engine, based on saidcurrent throttle position; and,

calculating an injector ON time for each cylinder based on said percentfull load value whereby, in use, the correct amount of gaseous fuel canbe injected into each cylinder responsive to the current throttleposition to achieve optimal engine performance.

Typically the method also comprises detecting the current speed of theengine and employing the current engine speed in said step ofcalculating the percent full load value. Preferably the percent fullload value is employed, together with the current engine speed, tocalculate a required manifold pressure value. The calculated value ofrequired manifold pressure may then be employed, together with thepercent full load value, in calculating a percent allowable load value.

Advantageously the position of a manifold valve in the engine intakemanifold is calculated for controlling manifold pressure based on saidcalculated percent full load value and detected engine speed.Advantageously a pressure detector is provided for detecting the actualmanifold pressure. If the engine is a turbocharged engine the positionof a boost pressure control valve is preferably controlled based on acomparison of said calculated value of the required manifold pressurewith the detected actual manifold pressure. Typically both said requiredand actual manifold pressure are absolute pressure values.

According to another aspect of the present invention there is providedan engine control unit for controlling the operation of a gas fuelledinternal combustion engine, the engine having a gas injector forinjecting gaseous fuel into each cylinder, the system comprising:

throttle position detector means for detecting the current throttleposition of the engine;

processor means for calculating a percent full load value for the enginebased On said current throttle position, and for calculating an injectorON time for each cylinder based on said percent full load value whereby,in use, the correct amount of gaseous fuel can be injected into eachcylinder responsive to the current throttle position to achieve optimumengine performance.

Typically the system further comprises speed detector means fordetecting the current speed of the engine, and said processor means alsoemploys current engine speed to calculate said percent full load value.Preferably said percent full load value is employed, together with theengine speed, to calculate a required manifold absolute pressure value.

Typically the system further comprises gas pressure and gas temperaturedetectors for detecting the pressure and temperature of gaseous fueldelivered to the engine respectively, and said processor means employsthe detected gas pressure and temperature in calculating the injector ONtime.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a better understanding of the nature of theinvention a preferred embodiment of the ECU will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a functional block diagram of a gas fuelled internalcombustion engine to which an embodiment of the ECU of the presentinvention is applied;

FIG. 2 is a flow chart illustrating the operation of a preferredembodiment of the ECU according to the invention;

FIG. 3 illustrates graphically the relative efficiency of a converteddiesel engine employing an ECU according to the invention, compared withthe original diesel engine performance as a function of load and enginespeed;

FIG. 4 is a three-dimensional chart which illustrates the variation inInjector On Time (IOT) as controlled by the preferred embodiment of theECU according to the invention;

FIG. 5 is a three-dimensional chart which illustrates the variation inManifold Valve (MV) position as controlled by the ECU;

FIG. 6 is a three-dimensional chart which illustrates the variation inManifold Absolute Pressure (MAP) position as controlled by the ECU;

FIG. 7 is a three-dimensional chart which illustrates the variation inSpark Advance (SA) as controlled by the ECU; and,

FIG. 8 is a graphical representation of a typical Percent Full Load(PFL) vs. Speed curve for an engine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a spark ignited internal combustion engine isillustrated diagrammatically at 10. The engine 10 is capable ofoperating on a gaseous fuel, and is provided with a gas delivery system12 for controlling the delivery of gaseous fuel from a gas reservoir 14,via a gas delivery line 16 to a fuel inlet 18 of the engine. The gasdelivery system 12 typically comprises individual solenoid operated gasinjectors (not illustrated) for each cylinder of the engine. Gasdelivery system 12 is under the control of an Engine Control Unitcontroller 20 via control line 22. The gas injectors of the gas deliverysystem 12 inject gas sequentially to the engine, each injectordelivering gas during the intake stroke of it's respective cylinder. TheEngine Control Unit (ECU) controller 20 delivers a signal via ignitioncontrol line 24 to a spark ignition system 26, which causes the fuelintroduced into the engine, along with air, to be ignited and henceprovide the power stroke of each cylinder.

Air to support combustion is delivered to the engine 10 through airinlet 28, for example, the inlet manifold of the engine, via airthrottle means 30. Air throttle means 30 may comprise, for example, amanifold valve which is also under the control of the ECU controller 20via manifold valve control line 32, for controlling the manifoldabsolute pressure. It may also be desirable to have some form offeedback means provided (not illustrated) for the manifold valve toprovide a feedback signal to the ECU controller 20, indicative of anoperating position of the manifold valve.

Air is delivered to inlet 28 via the manifold valve 30 from aturbocharger 34. Air at ambient pressure is supplied to the turbocharger34 through an air delivery line 36 from an air filter 38, which is opento atmosphere. Turbocharger 34 boosts the pressure of air delivered tothe engine 10 via manifold valve 30, above an ambient pressure. Sensor40a senses the air temperature in the boost air delivery line 42 andprovides a feedback signal indicative of the temperature to the ECUcontroller 20 via control line 44a. Sensor 40b senses the actualmanifold air pressure (MAP) and provides a feedback signal indicative ofthe MAP to the ECU controller 20 via control line 44b.

A boost pressure control valve 46 is provided in the boost air deliveryline 42 for controlling the boost pressure of air delivered to theengine 10. The boost pressure control valve 46 is directed to controlboost pressures under the control of ECU controller 20 via boostpressure control line 48. The boost pressure control valve 46 istypically provided with feedback means for providing a feedback signalto the ECU controller 20, indicative of an operating condition of theboost pressure control valve 46.

In this embodiment, the primary inputs to the ECU controller 20 areengine speed and throttle position. Engine speed is detected by a sensor50 which generates a signal indicative of the engine speed on input line52 to the ECU controller 20. Sensor 50 is typically an inductive (magnetand coil) sensor positioned adjacent to the ring gear teeth of the flywheel of the engine 10. The sensor produces a sinusoidal wave formhaving voltage and frequency characteristics that vary with the enginespeed. The actual engine speed is calculated by ECU controller 20 bymeasuring the time between a fixed number of peaks and troughs in thewave form.

Throttle position is measured by a throttle position sensor 54 whichprovides an electrical signal indicative of the throttle position oninput line 56 to the ECU controller 20. The throttle position sensor 54in this embodiment comprises a potentiometer "geometrically" coupled tothe throttle/accelerator pedal linkage. Typically, 0.0 Volts equates tozero position, whilst 5.0 Volts equates to maximum throttle position.

In the illustrated embodiment the engine 10 is a six cylinder engineprovided with three ignition coils (three coil pack, in which each coilfires two plugs simultaneously). A timing pulse is generated by a timingor engine phase sensor 58 mounted on the engine every second revolutionof the engine. Fly wheel teeth signals from the engine speed sensor 50are modified by a phase locked loop circuit which multiplies the signalfrequency by twelve. In this way two revolutions of the engine canalways be divided up into six intervals of equal angular displacement.Since the actual physical location of the timing sensor 58 is known, itis possible to nominate ignition timings in the standard units of °Crank Angle Before. Top Dead Centre. In this embodiment the ECUcontroller 20 also uses the 1/12 of a flywheel tooth interval(approximately 0.2°) as the increment by which spark advance may bevaried.

The ECU controller 20 typically comprises a microprocessor based controlsystem, having analogue to digital converters (ADC) for converting theanalogue signals from sensors 40a, 40b, 50 and 54, as well as thefeedback signal from the boost pressure control valve 46, into a digitalformat. Digital signals are employed to provide suitable control signalsfor controlling the manifold valve position, boost pressure controlvalve position and the operation of the gas delivery system 12 and sparkignition system 26. ECU controller 20 also comprises suitable read onlymemory (ROM) devices for storing look-up tables of engine operatingparameters, employed by the ECU controller 20 to achieve optimum engineperformance. The programmable nature of the ECU controller 20 means thatit can be readily modified to suit different engines, and providescomplete freedom in setting engine performance characteristics. As theengine is fully under the control of the Engine Control Unit responsiveto engine speed and throttle position as set by the driver (assuming theengine is a vehicle engine), the vehicle can effectively be under"drive-by-wire" control.

A method of controlling the operation of the gas fuelled internalcombustion engine 10, employing the Engine Control Unit described above,will now be described in detail with reference to FIG. 2.

On the left hand side of the flow chart of FIG. 2 each of the engineoperating parameters detected by the Engine Control Unit described aboveare listed, namely, engine speed 60, throttle position 62, manifoldabsolute pressure (MAP) 64, gas pressure 66, gas temperature 68, batteryvoltage 70, air temperature 72, engine phase 74 and boost pressurecontrol valve (BPCV) feed back position 76. Each of these parameters isprovided as an input in the form of an electrical signal to the ECUcontroller 20 of FIG. 1, for controlling the operation of the gasfuelled internal combustion engine 10. ECU controller 20 typicallyemploys the detected engine speed 60 and throttle position 62 tocalculate a percentage full load (PFL) value for the engine. PFL is anon-dimensional measure of the load or torque produced by or requiredfrom the engine. It is generally directly proportional to throttleposition, except in the governing regions of the load/speed envelope,where speed is controlled (see FIG. 8). PFL effectively indicates thevalue of the load required from the engine as a percentage of the fullload for a particular engine speed. It provides a way of determining theload required from or produced by the engine without having to measureor calculate the actual load, which is difficult in an on-roadsituation. Since PFL is non-dimensional, the absolute load value is notrequired and therefore the system can be applied to any engine. PFL iscalculated as follows:

If throttle=0%, and speed is <N_(i), then PFL=K (N_(i) -N), if speed is>N_(i), then PFL=0

If throttle is >0%, ##EQU1## Where N=engine speed

N_(i) =engine idle speed

N_(max) =maximum engine speed in governing region

N_(p) =maximum power engine speed

K=idle governing slope

PFL may also be adjusted to reduce engine output if the air inlettemperature goes too high, for example, due to a blocked intercooler (aheat rejection device located between the turbocharger air outlet andthe intake manifold). PFL may be reduced, for example, as follows:

    __________________________________________________________________________    Air Temp.                                                                             30° C.                                                                     40° C.                                                                     50° C.                                                                      60° C.                                                                     70° C.                                                                      80° C.                                                                     90° C.                               __________________________________________________________________________    Max Allowable                                                                         100 100 97   94  90   86  82                                          PFL                                                                           __________________________________________________________________________

the ECU controller 20 to calculate the injector On time (IOT) for eachgas injector in the gas delivery system 12. IOT is looked up in alook-up table stored in ECU controller 20, as a function of engine speedand load (PFL). IOT is determined empirically across the full range ofengine speeds and PFL by testing the engine in a test bed to produce thedesired output across the speed-load (PFL) envelope. The values for IOTare then stored in a ROM in the form of a look-up table. FIG. 4 is athree dimensional representation of the typical variation in IOT acrossthe speed-load envelope of the engine of the described embodiment.

ECU controller 20 also adjusts the timing of injector ON time (IOT) asthe engine speed increases, ie., the centre line of injection angle (IOTconverted to angle) is advanced as engine speed rises. This adjustmentis made to take into consideration the nominal opening time (ie., inwhich no gas flows) of the injector and the transport time to get gasfrom the injector to the inlet port. Again 1/12 of a flywheel tooth isused by the ECU controller as the increment by which the centre line ofinjection angle is varied.

The detected current engine speed 60 and the PFL calculation 78 may alsobe employed to calculate the manifold valve (MV) position of the intakemanifold valve 30 of the engine. MV is also established during enginetesting across the speed-load envelope, and the values stored in a ROMin the form of a look-up table. FIG. 5 illustrates in three dimensionalform the variation of MV across the speed-load envelope of the engine10. MV is looked up by ECU controller 20 as a function of engine speedand PFL. PFL is used since it ensures the manifold valve is opened asearly as possible during transients, which demand high loads. If someform of feedback means is provided in connection with the manifoldvalve, to provide a feedback signal to the ECU controller, the positionof the manifold valve can be even more accurately controlled. Suchfeedback means may include, for example, a position sensor connected tothe manifold valve, or the actual MAP may be employed to provide anindication to the ECU controller of the position of the manifold valve.This would avoid the need to establish the position of manifold valve atstart-up by moving the valve to its fully open and fully closedposition.

The current engine speed 60 and PFL calculation 78 may also be employedby ECU controller 20 to calculate the required manifold absolutepressure (MAP) in the intake manifold of the engine 10. The required MAPis determined empirically by experimentation/design as a function ofengine speed and PFL, to enable sufficient engine output to be achieved.For high engine output/torque a high MAP is required, conversely for lowengine output a low MAP is required. The required MAP values are alsostored in a ROM in the form of a look-up table, which is referenced byECU controller 20 to calculate the required MAP based on detectedcurrent engine speed 60 and calculated PFL 78. FIG. 6 illustrates inthree dimensional form the variation in required MAP across thespeed-load envelope of the engine 10.

If the engine 10 is turbocharged, as in the above described embodiment,the required MAP 82 is compared with the actual MAP 64, and afterallowing for an offset to allow for zero load MAP, a percent allowableload (PAL) is calculated at 84. PAL is required in the case ofturbocharged engines to prevent excess fuelling, as would occur if thegas delivery was only controlled by PFL. During transients some lagoccurs as the engine accelerates, since the turbocharger requires sometime to accelerate to the new operating state and to develop therequired MAP/boost pressure. PAL is calculated as follows: ##EQU2##wherein minimum MAP=representative value of MAP at PFL=0, and the termin brackets is clipped to be ≦1.00.

Typically, the calculated PAL 84 is used together with the currentengine speed 60 to look up the IOT from the IOT look-up table, ratherthan the calculated PFL 78 directly.

If the actual MAP exceeds atmospheric pressure and is higher than therequired MAP, the boost pressure control valve (BPCV) is actuated tocontrol the pressure, ie., the BPCV is opened to lower the boostpressure. Conversely, the BPCV is closed if the actual MAP is less thanthe desired MAP. The BPCV feedback signal 76 is employed by the ECUcontroller 20 to check correct BPCV operation. The actual position ofthe BPCV is not used in a control sense, but is used to check that thevalve has moved a discreet amount when commanded to move by ECUcontroller 20. If not, a fault is registered which indicates that theBPCV requires checking and/or servicing.

IOT is calculated at 80 using the detected current engine speed 60 andthe calculated PAL 84 for reference conditions of air temperature (AT)72, gas pressure (GP) 66, gas temperature (GT) 68 and battery voltage(BV) 70. If the detected values of these operating parameters differfrom the reference settings, corrective action is instigated by ECUcontroller 20 to alter the IOT and hence the amount of gas otherwisedelivered to the engine. In particular, if the air temperature risesabove it's reference set point, the mass of gas injected into the engineis reduced by 0.5% for each 3° C. rise above the reference temperature(e.g. 298° K.). To cater for variations in gas pressure and temperaturewhich effect gas density and sonic velocity in the orifice of a gasinjector, the following correction is applied to the effective injectoron time: ##EQU3## where, IOTE_(R) =injector ON time effectivereference=IOT-injector opening time at reference battery voltage.

GP_(R) =reference gas pressure

GT_(R) =reference gas temperature

AT_(R) =reference air temperature, and AT≧AT_(R).

GT_(R) is typically set at 15° C., which coincides with gas industrystandards for the measurement of gaseous fuel properties. It is also inapproximately the middle of the range for measurement of the actual gastemperatures. GP_(R) depends on the gas injectors employed, and thetrade-off between having a low gas pressure but long ON time and ahigher gas pressure with shorter ON time. GP_(R) is typically in therange of 700 to 800 kPa Gauge. A gas injector takes a discrete intervalof time to open, depending on the available battery voltage (BV) and thegas pressure (GP). The IOTE_(R) is therefore indicative of the "flowing"time the gas injector experiences and is therefore the IOT value thatadjustments are made to for gas pressure and temperature variations toobtain the effective injector ON time (IOTE).

The detected current engine speed 60 and calculated PAL 84 are alsoemployed by ECU controller 20 to calculate the spark advance (SA) 88 forthe reference air temperature 72. SA is also calculated by reference toa look-up table stored in ECU controller 20. FIG. 7 illustrates in threedimensional form the variation in SA across the speed-load envelope. Ineach of FIGS. 4, 5, 6 and 7 all the values indicated are non-dimensionaldigital values used by the software resident in the ECU controller 20,with the exception of engine speed which is in units of RPM. SA is alsodetermined by testing the particular engine for which the Engine ControlUnit is designed. SA is required due to the small delay (order 2milliseconds) which occurs before combustion starts after the initiationof a spark. This delay varies with changes in air temperature 72 andgas/air ratio. The higher the air temperature, the less delay andconversely for lower air temperatures. Therefore, an adjustment is madeto the value of SA derived from the look-up table based on the detectedair temperature 72. An engine phase marker 74 (or timing signal) is usedas the datum for setting SA. The engine phase marker 74 is also employedto set the timing for the correct sequencing 80 of the gas injectors.

Now that the operation of the preferred embodiment of the Engine ControlUnit according to the invention has been described in detail, it will beapparent that the system has significant advantages over prior artsystems for controlling the operation of gas fuelled internal combustionengines. In particular, by employing gas injectors and accuratelycalculating the injector ON time the correct amount of gaseous fuel canbe injected into each cylinder to achieve optimum engine performanceunder the full range of engine speed and load conditions. Thusoverfuelling can be avoided and significant fuel economies achieved,FIG. 3 illustrates graphically the relative efficiency of a converteddiesel engine employing an Engine Control Unit according to theinvention, compared with the original diesel engine performance as afunction of engine load and speed. From FIG. 3 it can be seen that theconverted engine achieves better than 80% relative efficiency over mostof the speed-load envelope, and greater than 90% relative efficiency atengine loads below 300 Nm.

Numerous variations and modifications will suggest themselves to personsskilled in the mechanical engineering arts, in addition to those alreadydescribed, without departing from the basic inventive concepts. Forexample, the Engine Control Unit according to the invention is alsoapplicable to a gas fuelled internal combustion engine which is notturbocharged. The system and method according to the invention are alsoapplicable with suitable modifications to dual fuelled engines. All suchvariations and modifications are to be considered within the scope ofthe present invention, the nature of which is to be determined from theforegoing description and the appended claims.

I claim:
 1. A method of controlling the operation of a gas fuelledinternal combustion engine, the engine having a gas injector forinjecting gaseous fuel into each cylinder, the method comprising thesteps of:detecting the current throttle position of the engine;calculating a percent full load value for the engine, based on saidcurrent throttle position; and, calculating an injector ON time for eachcylinder based on said percent full load value whereby, in use, thecorrect amount of gaseous fuel can be injected into each cylinderresponsive to the current throttle position to achieve optimal engineperformance.
 2. A method of controlling the operation of a gas fuelledinternal combustion engine as claimed in claim 1, further comprising thestep of detecting the current speed of the engine and employing thecurrent engine speed in said step of calculating the percent full loadvalue.
 3. A method of controlling the operation of a gas fuelledinternal combustion engine as claimed in claim 2, wherein the percentfull load (PFL) value for the engine is calculated as follows:Ifthrottle=0%, and speed is <N_(i), then PFL=K (N_(i) -N), if speed is>N_(i), then PFL=0. If throttle is >0%, ##EQU4## Where N=engine speedN_(i) =engine idle speed N_(max) =maximum engine speed in governingregion N_(p) =maximum power engine speed K=idle, governing slope.
 4. Amethod of controlling the operation of a gas fuelled internal combustionengine as claimed in claim 3, wherein the percent full load value isemployed, together with the current engine speed, to calculate arequired manifold absolute pressure value.
 5. A method of controllingthe operation of a gas fuelled internal combustion engine as claimed inclaim 4, further comprising the step of calculating a percent allowableload (PAL) value based on the calculated values of the required manifoldabsolute pressure (MAP) and the PFL.
 6. A method of controlling theoperation of a gas fuelled internal combustion engine as claimed inclaim 5, wherein the percent allowable load (PAL) value for the engineis calculated as follows: ##EQU5## wherein minimum MAP=representativevalue of MAP at PFL=0, and the term in brackets is clipped to be ≦1.00.7. A method of controlling the operation of a gas fuelled internalcombustion engine as claimed in claim 6, the injector ON time (IOT) iscalculated using the calculated value of PAL and the current enginespeed.
 8. A method of controlling the operation of a gas fuelledinternal combustion engine as claimed in any one of claims 1 to 7,wherein the injector ON time (IOT) is corrected for variations fromreference conditions of air temperature (AT), gas pressure (GP), gastemperature (GT) and battery voltage (BV) to obtain an effectiveinjector ON time (IOTE) as follows: ##EQU6## where, IOTE_(R) =injectorON time effective reference=IOT-injector opening time at referencebattery voltage.GP_(R) =reference gas pressure GT_(R) =reference gastemperature AT_(R) =reference air temperature, and AT≧AT_(R).
 9. Amethod of controlling the operation of a gas fuelled internal combustionengine as claimed in claim 4, wherein the position of a manifold valvein the engine intake manifold is calculated for controlling manifoldpressure based on said calculated percent full load value and detectedengine speed, and wherein a pressure detector is provided for detectingthe actual manifold absolute pressure.
 10. A method of controlling theoperation of a gas fuelled internal combustion engine as claimed inclaim 9, wherein the engine is a turbocharged engine and the position ofa boost pressure control valve is controlled based on a comparison ofsaid calculated value of the required manifold absolute pressure withthe detected actual manifold absolute pressure.
 11. An engine controlunit for controlling the operation of a gas fuelled internal combustionengine, the engine having a gas injector for injecting gaseous fuel intoeach cylinder, the system comprising:throttle position detector meansfor detecting the current throttle position of the engine; processormeans for calculating a percent full load value for the engine based onsaid current throttle position, and for calculating an injector ON timefor each cylinder based on said percent full load value whereby, in use,the correct amount of gaseous fuel can be injected into each cylinderresponsive to the current throttle position to achieve optimum engineperformance.
 12. An engine control unit as claimed in claim 11, furthercomprising speed detector means for detecting the current speed of theengine, and said processor means also employs current engine speed tocalculate said percent full load value.
 13. An engine control unit asclaimed in claim 12, wherein said processor means further comprisesmeans for calculating a required manifold absolute pressure value basedon said percent full load value and the current engine speed.
 14. Anengine control unit as claimed in claim 13, further comprising pressuresensing means for sensing the actual manifold absolute pressure of airdelivered to the engine, and providing a feedback signal to theprocessor means indicative of the actual manifold absolute pressure. 15.An engine control unit as claimed in the claim 14 wherein the engine isturbocharged and the control unit further comprises a boost pressurecontrol valve for controlling the boost pressure of air delivered to theengine responsive to a control signal from the processor means, saidcontrol signal being generated based on a comparison of the calculatedrequired manifold absolute pressure and the sensed actual manifoldabsolute pressure.
 16. An engine control unit as claimed in claim 15,wherein said processor means further comprises means for calculating apercent allowable load (PAL) value based on the calculated values of therequired manifold absolute pressure (MAP) and the PFL.
 17. An enginecontrol unit as claimed in claim 16, wherein said means for calculatingthe PAL value for the engine employs the following formula: ##EQU7##wherein minimum MAP=representative value of MAP at PFL=0, and the termin brackets is clipped to be <1.00.
 18. An engine control unit asclaimed in claim 17, wherein the processor means calculates the injectorON time (IOT) using the calculated values of PAL and the current enginespeed.
 19. An engine control unit as claimed in claim 18, furthercomprising gas pressure and gas temperature detectors for detecting thepressure and temperature of gaseous fuel delivered to the enginerespectively, and said processor means employs the detected gas pressureand temperature in calculating the injector ON time.